Medical articles are conventionally made by thermoplastic processing of polyolefins or copolymers thereof. Typical plastics used are polyethylene, polypropylene or copolymers of these materials with one or more other monomers such as styrene, acrylic or butadiene. A variety of physical properties can be achieved according to the medical application envisioned for the plastic.
One important criterion to be considered when selecting a plastic for medical application is the morphological stability of the plastic when subjected to physical stress, such as conventional sterilization procedures. Thus, the plastic must not undergo any substantial change in size or shape during sterilization which would compromise the specifications required for proper end use. This is particularly true for multicomponent articles having contiguous parts of close tolerances where avoidance of size changes is very important if sealing contact between the parts must be maintained.
If the sealing components also have a sliding relationship, the phenomenon known as stick-slip is often a problem. Thus, it is well known that two plastic surfaces having a sliding relationship often exhibit sufficient resistance to initiation of sliding movement that gradually increased pressure applied to one of the surface does not cause movement until a threshold pressure is reached at which point a sudden sliding separation of the surfaces takes place. This situation is commonly referred to as stick-slip or sticktion. Stick-slip is exacerbated by prolonged stationary contact between the surfaces, such as occurs during shelf time. It is particularly troublesome in devices such as syringes, in particular syringes to be used with syringe pumps where ultra slow advancement of the stopper is required, and repeating sequential episodes of stick-slip occurs. The term tribology refers to the study of friction, lubrication and wear of surfaces in relative motion.
Natural rubber has been fabricated into articles of low stick-slip which resist morphological change because of low compression set. This material is, however, expensive, difficult to process and for this reason, SANTOPRENE.TM. (blend of polypropylene and EPDM (ethylene-propylenediene monomer) has become a material of choice for stoppers intended for medical use. This material, while of excellent compression set, is subject to substantial stick-slip.
It is toward provision of articles, particularly multicomponent medical articles of improved set and seal retention and minimal stick-slip that the present invention is directed.